When the Lights Go Out

D. Caroline Coile, Ph.D.

"He’s blind." Blind? Except for a peculiar gait, the young Ibizan hound
seemed as active and inquisitive as the other dogs cavorting in the yard.
Still, I had to wonder if it was really fair to raise a blind dog---and a
sighthound at that. "You ought to see him run…" his owner continued, "even
through the woods." What? Surely this was some kind of dog abuse. Or was
the dog really blind? How would you tell? What could have caused him to
lose his vision? And just how handicapped is a blind dog?

Blind Faith

Most humans who lose their vision face extensive rehabilitation in order to
function independently. As a result, people tend to think of blind dogs as
similarly, or even more severely, affected. In a survey of 50 owners of
blind dogs, half of the owners reported that they had been chastised by
friends who considered it cruel to keep a blind dog. 1 The owners of these
dogs, however, would tend to disagree.

Most of the blind dogs still led full and active lives. Most still enjoyed
going for walks. Surprisingly, about half of the dogs were taken for walks
off leash (except when next to a road), with the owners expressing
confidence that the dog was safe. Two dogs were even allowed to roam at
will (one was killed by a car). Six owners reported their dogs could still
play ball by relying on their other senses. The dogs could recognize
frequent visitors by scent.

This is not to say that the dogs were unaffected by their blindness.
Owners reported having made considerable efforts to contain their dogs in
familiar environments. They avoided moving furniture within the home or
boarding the dog at a kennel. Despite all efforts, twelve percent of dogs
never learned to adequately cope even in familiar surroundings. Roughly
half of the owners reported permanent behavioral or temperament changes in
their dogs, mostly in the form of greater timidity, caution, and dependence
upon the owner. Dogs would tend to walk rather than run, and some dogs
would bark for attention. The ability of dogs to cope successfully was not
related to their breed or size. Dogs that lost their vision relatively
suddenly tended to have greater behavioral changes. Two dogs that had
become blind within three months became aggressive. In general, two thirds
of the owners felt their relationship with their dog had changed as a
result of its blindness, with increased dependence of the dog on the owner,
and ultimately a closer relationship.

Eyeing the Causes of Blindness

The complexity of the eye led Charles Darwin, in his Origin of Species, to
reflect that its development was one of the most difficult challenges to
his theory of evolution. Its function relies upon the interplay of
mechanical, optical, chemical, and neural aspects that are themselves
extremely complex. That vision is possible at all is nearly
incomprehensible; that it is fragile should not be surprising.
The list of visual disorders potentially leading to loss of sight is a long
one. More hereditary disorders of the eye have been identified than for
any other organ of the dog. These disorders can affect virtually every
structure of the eye. In addition to trauma and infection, the leading
causes of blindness are cataracts, glaucoma, uveitis, corneal erosion, and
tumors. 2

Cataract, in which the normally transparent lens becomes opaque, is the
leading single cause of blindness, especially in older dogs. Cataracts
occurring in younger dogs (less than six years of age) most often have a
hereditary component, but may also result from diabetes, nutritional
deficiency, toxins, or injury. It’s normal for older dogs to have a slight
haziness to the lens, but when the haze becomes nearly white, vision is
impaired. Fortunately, cataractous lenses can be removed and replaced with
prosthetic lenses, just as in humans.

Glaucoma is a condition in which the intraocular pressure of the eye is
raised to the point that injury to the eye (specifically the retina and
optic nerve) occurs. The normal eye has a constant influx of aqueous fluid,
which fills the areas between the cornea and lens. The fluid is normally
drained away at the same rate new fluid is introduced, the main outlet
being through the iridocorneal angle (where the outer edges of the iris and
cornea meet). When this angle is closed, abnormally narrow, or otherwise
compromised, the fluid is inadequately drained. Such problems can arise
through hereditary factors, injury, lens displacement, inflammation, and
tumors, among other causes. As the intraocular pressure due to increased
fluid builds up, the eye becomes swollen, red, and diffusely painful, the
pupil becomes dilated and unresponsive, the cornea may become clouded, and
permanent blindness can result within hours. If detected early, drugs, and
especially surgery, can often save vision, but long-term success is
disappointingly low.

Uveitis is inflammation of the eye, most specifically the iris, ciliary
body, and choroid (the layer of the eye that contains the vascular
supply). It can result from many causes, including infectious, fungal, and
tick borne diseases, immune-mediated disorders, and neoplasias. Symptoms
can include pain, redness, sensitivity to bright light, tearing, and
constriction of the pupil. If left unchecked, uveitis can cause glaucoma,
retinal scarring or detachment. It can be treated medically, but often
owners are unaware of the problem until secondary problems have made their
appearance.

Corneal erosion can result when small corneal abrasions are not treated.
Such abrasions may result from infection, chemical injury, metabolic or
immunological factors, an abnormally dry eye, or constant irritation from
lids, lashes, or hair. When the ulcer becomes so advanced that all the
layers of the cornea are eroded, the eye can rupture and blindness can
eventually result. Early symptoms may include tearing, mucous discharge,
squinting, pawing at the eye, and sometimes, an obvious dullness to or
marring of the corneal surface. Medication or surgery may effect a cure.

Tumors of the eyelid, third eyelid, cornea, iris, and all along the
visual pathway, can eventually result in blindness. The nature, size, and
location of such tumors will determine whether they become
sight-threatening. Many can be surgically removed.

The list of ocular and visual diseases is much longer, and includes ---but
is not limited to---such well-known or common disorders as entropion and
ectropion (in which the eye lid is rolled inward or outward, respectively),
distichiasis and trichiasis (in which eyelashes grow from an abnormal lid
site or are misdirected, respectively), keratoconjunctivitis sicca (KCS,
or dry eye due to decreased tear production), corneal dystrophy (in which
opacities develop around the central area of the cornea), pannus (in which
vascularized pigment grows over the cornea) , luxated or subluxated lens
(in which the lens attachment is compromised, leading to a lens that is
totally or partially displaced, respectively), progressive retinal
atrophies (PRA, a group of retinal disorders in which the photoreceptors
either develop abnormally, incompletely, or once developed, degenerate),
retinal detachment (in which the retina tears and becomes partially
detached from its underlying layers) , and Collie Eye Anomaly (in which
several layers of the eye develop abnormally, leading to blind spots of
varying sizes) , most of which can also threaten vision.

Dark Suspicions

Many owners of blind dogs report that their dogs seem to have suddenly lost
their vision, but in most such cases the dog more likely gradually lost its
vision over a period of time until it finally reached a point at which it
could no longer function normally. Often the dog had been exhibiting subtle
cues that had gone unnoticed by its owner. These include temperament
changes (usually becoming more fearful or aggressive), greater dependence
on the owner, lethargy, weight gain, a cautious, often high-stepping gait,
and even a tendency to drink large quantities of water due to psychological
factors (called psychogenic polydipsia). Sometimes more specific problems
will be noted by the owner, such as impaired ability to chase balls, climb
steps or curbs, jump up or off of furniture or high places, notice objects
on one side but not on the other, or objects nearby but not far away.
By performing some simple behavioral tests, an owner can get a better idea
of whether a dog may be visually impaired. Several visually guided reflexes
can be tested fairly simply:

Bright light eye blink reflex: When a bright light is shone in either
eye, both eyes will normally blink. The pathway is subcortical, meaning that it does not involve conscious perception of light. Thus,
a dog with a normal bright light eye blink reflex can still be essentially
blind if it has a problem affecting the brain’s visual cortex area (or the
pathways to that area).

Pupillary light reflex: When a concentrated bright light is shone into
one eye, the pupil of that same eye will constrict (the direct pupillary
light reflex), as well as the pupil of the other eye (consensual pupillary
light reflex). The absence of pupillary constriction in an otherwise normal
dog usually points to a problem of the eye (including glaucoma, iris
sphincter muscle defect, or retinal disease), optic nerves, optic chiasm,
or optic tracts. A pupil that is constricted and unresponsive to light can
indicate uveitis as well as several other possible causes. When pupillary
responses are normal, yet the dog appears blind, the problem is usually in
the brain. Seldom, however, is the delineation so clear-cut, since only a
few functioning retinal receptors are needed to subserve a pupillary
reflex, yet those few may not be sufficient to sustain vision. 3

Menace reflex: When an object threatens to touch the eye, the natural
response is to blink. The easiest way to test this reflex is by moving a
hand or other object quickly toward the eye; however, the air currents
created by such a movement can also cause a blink reflex. Thus, a sheet of
clear glass or plastic should be placed between the dog and the moving
object when testing. Another solution is to move the hand from above the
head downward in a plane parallel to the eye.

Visual placing reflex: When a dog is picked up and carried toward a
surface (such as a raised table), it will naturally reach forward with its
front legs and try to place them on that surface, guided by sight.

Optokinetic nystagmus: When a series of objects are moved rapidly from
one side of the visual field to the other, the eye will rhythmically
oscillate back and forth. This response can be elicited in humans by
spinning a drum with black and white vertical stripes in front of the eyes,
but it can be more difficult to elicit in dogs this way. This probably
simply reflects difficulty in getting a dog to look at the drum in the
first place. Some innovative researchers solved the problem by placing
rabbits on a rotating platform in front of dogs---a technique that
certainly got their attention! 4

Other behavioral tests involve more than simple reflex pathways. These
tests should be repeated under both bright and dim lighting conditions. For
dim lighting, the dog should first be allowed to adjust to the dark for
about 15 minutes. A blindfold can be placed over one eye at a time so that
each eye can be tested separately.

One such test is to throw an object and see if the dog tracks it visually.
The problem with throwing a ball or piece of food is that they either make
a noise when they hit the ground or have a strong odor. The solution is to
use a cotton ball. Most dogs will attend to this initially, but often
become quickly bored with this test so the dog’s response may become
subsequently diminished. Another simple behavioral test is to call the dog
through an obstacle course of soft furniture, waste paper cans, or other
items.

Illuminating Opinions

Simple screening tests are not very sensitive, so will probably only
uncover dogs that have severe visual deficits. An owner who suspects a
visual problem can perform these primitive screening tests at home, but
will ultimately want to get the opinion of a veterinarian, and preferably,
a veterinary ophthalmologist.

A veterinary ophthalmologist can perform far more sophisticated tests as
well as a complete health examination and history. One of the most
important pieces of information is the dog’s breed, since so many visual
problems have a breed predilection. Color of the dog is also noteworthy;
some colors, such as dapple, merle, or white due to albinism can be
associated with sensory defects. Age of the dog is another important
factor, since many hereditary disorders have a fairly predictable age of
onset. Older dogs are prone to a number of visual problems less commonly
seen in younger dogs. Because some infectious diseases or toxins, as well
as trauma, can result in visual impairment, the dog’s health status,
environment, and lifestyle can also provide possible clues.

Simply looking at and in the eye can be suggestive of a visual problem.
Many ophthalmic problems involving the lids and cornea can cause tearing
and squinting, and such problems, if left unchecked, can eventually result
in loss of vision. A pupil that is perpetually dilated, does not respond
to light, or does not match its fellow pupil can be a sign of ophthalmic or
neurological problems. A displaced lens may be visible through the pupil,
and can also cause blindness if not treated.

Windows to the Soul

Most visual problems are not so outwardly apparent, however. Glaucoma may
be suspected from outward signs, but should be diagnosed with a tonometer
to record the intraocular pressure and a gonioscope to visualize the
iridocorneal angle. An ophthalmoscopic examination is necessary to peer
into the eye, focussing on the lens or rear surface (called the fundus) of
the eye. The fundus has a characteristic pattern of blood vessels that lie
over the surface of the retina. Abnormal variations in the pattern of blood
vessels are readily apparent to the trained eye, and can also point to
specific retinal problems. By injecting fluoroscein into the bloodstream,
(a technique called fluoroscein angiography) retinal circulation can be
observed. Although an ophthalmoscope or lens is needed to visualize the
fundus clearly, the blood vessels are sometimes observable to the naked eye
if the dog’s pupils are dilated and light enters its eye along roughly the
same path as your own line of sight. Using the same technique, you can also
see a reflection from the tapetum lucidum from the upper half of the
fundus. Increased reflectivity of the tapetum can be indicative of retinal
disorders.

Observation of the dog’s behavior and eyes can provide much information
about a dog’s visual health, but more comprehensive electrophysiological
tests can often better characterize the nature of visual loss, and detect
it at a far earlier stage. The most clinically useful of these is the
electroretinogram (ERG).

When retinal cells respond to light, they cause a change in the electrical
potential of the eye that can be measured simply by placing an electrode on
the surface of the cornea. The drawback is that the dog must be
anesthetized in order to stay still and tolerate the contact lens
containing the electrode. The normal ERG has a particular shape and
consists of three distinctive waves. By performing ERGs under different
lighting conditions, the function of the rods and cones can be
independently evaluated. In some breeds, ERG screening of young puppies can
detect dogs that will later develop progressive retinal atrophy (PRA)
sometimes months to years before behavioral or ophthalmic signs are
present. The ERG is an essential test when considering a dog for cataract
removal when the lens is so opaque the examiner can’t see through it to
view the fundus. By shining an extremely bright light into the eye, it’s
possible to determine if the retina is functioning, since it would be of
little use to remove a cataractous lens if the retina could not respond to
light.

The Path to Enlightenment

A dog can have a normal fundus and ERG and nonetheless be blind or
partially blind. In such cases the results of the reflex tests can suggest
a neurological problem. In order to pinpoint the location or nature of such
a problem, the visual pathways must be understood.
Recall from previous articles 4,5 that light passes through the pupil and
lens to be focussed on the retina, which contains both rod and cone
receptors. These receptors transmit their responses to light by way of
several other cell types to the ganglion cells of the retina. The ganglion
cells in turn have long fibers, or axons, that all leave the retina at the
same place (the optic disc) and travel together as the optic nerve toward
the brain.

Before reaching the brain, the optic nerves from the two eyes join together
in the optic chiasm, which can be likened to a neural intersection. Here
some of the axons cross over to the opposite ("contralateral") side of the
brain, while other axons continue onward on the same ("ipsilateral")
side. This partial crossover may seem peculiar, but is a vital aspect of
binocular vision and depth perception. In an animal with forward placed
eyes, there is some overlap of the visual fields so that some parts of the
visual world are seen by both eyes. This crossing allows fibers that
receive information about the same point in visual space to project to the
same part of the brain. In a species with virtually no overlap of visual
fields (such as the rabbit, with laterally placed eyes) 100% of the fibers
cross; in a species with extensive overlap (such as humans, with frontally
placed eyes) 50% of the fibers (those from the nasal half of each retina)
cross. In dogs, with obliquely placed eyes, 70 to 75% of the fibers cross.
6 Whether or not this percentage changes in dogs with different head
configurations, and thus different degrees of binocular overlap, has not
been investigated (flat-faced "brachycephalic" breeds generally have more
frontally placed eyes than do long-nosed "dolichocephalic" breeds).
Reported values of binocular fields in dogs range from 116 degrees in a
"ratter" to 78 degrees in a setter. 7,8

After leaving the optic chiasm, the fibers continue on to the brain by way
of the optic tract, making contact with cells in three different parts of
the brain: lateral geniculate nucleus (LGN), superior colliculus, and
pretectal nucleus. Each of these centers is responsible for different
processing of the visual input. Most of the fibers project to the LGN,
which performs some neural processing and then in turn routes the signals
by way of the optic radiation to the visual cortex. This route is
considered the primary pathway responsible for conscious vision.
Visual loss due to neurological problems often results from injury to or
tumors on parts of the visual pathways. If a dog is blind in only one eye,
the problem must be in that eye or its optic nerve. If a dog is blind in
both lateral visual fields, so that it can only see in a small area that is
normally seen binocularly, a lesion that essentially splits the optic
chiasm, so that only the non-crossing fibers get through, must be
suspected. Such a problem can arise from a tumor impinging upon the
chiasm, most often a tumor of the pituitary gland, which is situated just
beneath the chiasm. If one side of the dog’s visual field is gone, so that
it is blind in the monocular segment of one eye and the binocular segment
of the other, the problem is most likely in the optic tract or higher
visual center of the brain. Using such processes of deduction, the
veterinary ophthalmologist or neurologist can then attempt to pinpoint the
exact area in the visual pathway using more sophisticated imaging
techniques.

Be on the Look-out

The eye is a sensitive organ that can easily be irreparably damaged. Its
complexity, combined with the difficulty to pinpoint small defects in
vision in an animal that cannot be tested with a standard eye chart, makes
the task of the veterinary ophthalmologist particularly challenging. Dogs
seldom give any clues that their vision is deteriorating until it is nearly
abolished. In many cases salvage of vision relies upon the astute
observation of the owner. A dog with any evidence of visual problems
should be seen by a veterinarian as soon as possible, in some cases even if
it means a middle of the night visit to the emergency clinic. Many
problems can be cured and vision saved if caught in time, but in some
cases---most notably glaucoma—that window of opportunity can be small.
Remember, "blindsight" is 20/20.

If a dog does lose its vision, it can still lead a full and happy life, as
long as the owner makes certain concessions. Dogs have a well-developed
sense of vision, and rely upon it to perform many of their services to
humans. Yet unlike humans, dogs that lose this vital sense seem to be able
to adjust with minimal rehabilitation. Perhaps this is because of the
greater sensitivity of their other senses, most notably their abilities to
hear and smell, senses that will be the topic of upcoming articles as we
delve deeper into the dog’s sensory world.

Self demonstrations: How to see your own blind spot and fundus
In a strange bit of design, the retinal axons travel over the front surface
of the retina, so that at the optic disc no receptors are present, forming
a blind spot. Most students have "seen" their own blind spot by means of a
simple demonstration: Place two marks on a piece of paper about four
inches apart. Close one eye at look at the mark on your nasal side with the
other. Move the paper slowly back and forth from about five to 15 inches in
front of your eye. At some point, when the image of the lateral mark falls
on your optic disk, it will disappear!

Note that unless you go to some lengths, you are never aware of your blind
spot because your brain "fills in" the gap with whatever happens to be
surrounding the spot. This is why one or two small areas of the retina (or
brain) affected by disease may not initially be apparent to you or your
dog—the brain is wonderfully adept at compensating for small areas of
visual loss.

Not only are the ganglion cells in front of the receptors, but also a
network of retinal blood vessels. You may be able to see your dog’s fundus
to some extent by taking it into a dim room so its pupils dilate, then
holding a penlight or small flashlight next to your own eye but aimed into
your dog’s pupil. You should see the iridescent shimmer of the tapetum, and
perhaps a few red blood vessels. How can the dog see "through" these blood
vessels? The same way we do; that is, a number of complex neural
mechanisms act to make us essentially "blind" to items that are constantly
there. If the light were to hit the blood vessels from a different
direction they could be made visible. In fact, you can see the blood
vessels in your own eye by closing your eye and looking toward your nose as
much as possible, then lightly pressing a pen light (wit the beam on) to
the outermost corner of your lid, just inside the temporal eye socket.

You can estimate your dog’s field of view by holding a tidbit directly
Jiggle the light a bit and behold your own fundus!
in front of him so that it attracts his attention, and then quietly and
slowly moving an object from behind your dog forward around the side of his
head, making note of the point at which it is first noticed. Another
simple method is to observe how far to the side of your dog you can move
until your dog’s pupils are no longer visible to you, which would indicate
the farthest possible extent of lateral vision.